Shrink the SkImageGenerator API

generateBitmap was used in one test, so it's easily converted to
tryGenerateBitmap. Then, all calls to tryGenerateBitmap supplied
an image info, so we don't need it to be optional.

BUG=skia:

Change-Id: I19e8f9da7e442a2d37af68b029b5ec85228766f7
Reviewed-on: https://skia-review.googlesource.com/6149
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed